Allow using docutil.get_inkscape_path() on Linux also.

Moved XSLT model query python code so that XSLT part can be reused for other transformations

Fixed unitialized parameter in instances_path stylesheet, leading to
calls to add_instance with no data when passing datatypes insteatd of project
root. That bug was making exceptions (for exemple when editing type) when
having arrays of derivated types in project.